System for recording digital information in a pulse-length modulation format

ABSTRACT

An improved system for recording and playing back digital information in a special pulse-length modulation format on a disc-shaped record. The digital information is stored in a succession of alternating marks and spaces, both having lengths that are discretely variable in accordance with a succession of multi-bit binary code blocks.

This is a division of application Ser. No. 08/180,312 filed Jan. 12,1994, U.S. Pat. No. 5,373,490, which is a continuation of 08/075,275,filed Jun. 11, 1993, now U.S. Pat. No. 5,321,680; which is acontinuation of Ser. No. 07/948,267 filed Sept. 21, 1992, U.S. Pat. No.5,253,244; which is a continuation of Ser. No. 07/825,640 filed Jan. 24,1992, abandoned; which is a continuation Ser. No. 07/645,638 filed Jan.25, 1991, U.S. Pat. No. 5,084,852; which is a continuation of Ser. No.07/499,217filed Mar. 16, 1990, U.S. Pat. No. 5,003,526; which is acontinuation of Ser. No. 06/782,156 filed Oct. 2, 1985, abandoned; whichis a continuation of Ser. No. 06/169,238 filed Jul. 16, 1980, abandoned.

BACKGROUND OF THE INVENTION

This invention relates generally to systems for storing digitalinformation, and, more particularly, to video disc systems for storingdigital information in a pulse-length modulation format.

Video disc systems are becoming widely used for storing digitalinformation with a high recording efficiency. The information isordinarily recorded on the disc as a succession of spaced marks arrangedin a plurality of substantially circular and concentric recordingtracks, for example, a spiral pattern. One particularly efficient systemhas recorded the digital information in a pulse-length modulationformat, in which each of the successive spaced marks has adiscretely-variable length representative of a separate, multi-bit codeblock. The spacing between successive marks, or alternatively thespacing between the beginning edges of successive marks, is ordinarilymaintained constant.

The vide disc can include a glass substrate, with a thin. metallicrecording layer overlaying it and apparatus for recording the digitalinformation on the disc ordinarily focuses a writing beam of light ontothe disc, as the disc is rotated at a uniform rate, with the intensityof the beam being modulated in accordance with the digital informationto be recorded. When the intensity exceeds a predetermined threshold, anon-reflective pit or mark is formed in the recording layer, whereaswhen the intensity does not exceed the threshold, the recording layer isnot affected. Thus, the lengths of the successive, spaced markscorrespond to the time duration the intensity of the focused beamexceeds this threshold.

The recorded digital information is recovered from replicas of therecorded video disc by scanning it with a reading beam of light having auniform intensity, to produce a reflected beam having an intensitymodulated by the recorded pattern of spaced marks. The length of timethe intensity of the reflected beam exceeds a predetermined level isthen measured to determine the length of the corresponding mark and thusthe particular binary code block it represents.

Although this prior pulse-length modulation technique has proveneffective in recording digital information with a relatively highrecording efficiency, there is still a need for a system for recordingdigital information with an even higher efficiency. The presentinvention fulfills this need.

SUMMARY OF THE INVENTION

The present invention is embodied in a system for recording and playingback digital information on a record medium, in which the information isstored in a succession of spaced marks of discretely-variable lengths.The length of each mark is representative of a separate one of asuccession of multi-bit binary code blocks. In accordance with theinvention, the spaces between successive marks also havediscretely-variable lengths representative of separate blocks in thesuccession of code blocks. Digital information is thereby recorded onthe record medium with a yet higher recording efficiency.

More particularly, the present invention has particular utility in avideo disc system in which a video signal is recorded on a disc-shapedrecord in a succession of substantially circular and concentricrecording tracks. The recording apparatus functions initially todigitize the video signal and to compress the digitized signal, usingknown data compression techniques. The digitized signal is then arrangedin a succession of code blocks of the same or mixed lengths, and abinary modulation signal is formed having transitions in statedetermined in accordance with the successive code blocks. In thepreferred embodiment, each code block includes four binary bits, and thesuccessive states of the modulation signal have sixteen possiblediscrete durations.

The modulation signal is coupled to a light intensity modulator, whichmodulates the intensity of a writing beam such that the intensity isalternately greater than and less than a predetermined threshold fortime durations corresponding to the succession of multi-bit code blocks.The intensity-modulated beam is focused onto the record, as the recordis rotated at a prescribed rate, to form corresponding microscopic pitsor marks in a prescribed pattern. Using conventional techniques, therecord can then be used to produce video disc replicas.

The recorded digital information is played back from disc replicas byscanning the successive tracks with a reading beam of light having asubstantiaIly uniform intensity. This produces a reflected (ortransmitted) beam having an intensity modulated by the recorded patternof alternating marks and spaces. The playback apparatus measures thetime durations of the successive marks and spaces and determines theparticular code blocks each represents. After de-compressing thesuccession of detected code blocks, the original analog video signal canbe re-created.

In-the preferred embodiment, each of the successive recording tracks isused to record a separate video frame. Since the special pulse-lengthmodulation format results in a recorded pattern of marks and spaceshaving a variable length, however, it is usually the case that less thanan entire track is required to record each video frame. When thisoccurs, the remainder of the track is occupied by alternating marks andspaces representative of a prescribed filler code.

Other aspects and advantages of the present invention will becomeapparent from the following description of the preferred embodiment,taken in conjunction with the accompanying drawings, which disclose, byway of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of recording apparatus inaccordance with the present invention, for recording a digitized videosignal on a video disc, in a special pulse-length modulation format;

FIG. 2 is a simplified block diagram of playback apparatus in accordancewith the present invention, for recovering the digitized video signalstored on the video disc in the special pulse-length modulation format;

FIG. 3 is a schematic diagram illustrating the format of the data on thevideo disc; and

FIG. 4 is a table showing the prescribed lengths for the sucessive marks(and spaces) corresponding to each of the plurality of possible 4-bitcode blocks being recorded.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, and particularly to FIG. 1, there isshown recording apparatus for recording a digitized video signal on adisc-shaped record 11. The apparatus includes a writing laser 13 forproducing a writing beam of light 15 having a substantially uniformintensity, and an intensity modulater 17 for modulating the intensity ofthe beam in accordance with a digital modulation signal to be recorded.The apparatus further includes a radially-movably objective lens (notshown) for focusing the intensity-modulated beam onto the record, and aspindle motor 19 for rotating the record at a prescribed, uniform rate(e.g., 1800 r.p.m.). The focused beam thereby forms a succession ofsubstantially circular and concentric recording tracks in the record.

The record 11 includes a glass substrate with a metallic recording layeroverlaying it, and the focused beam forms a microscopic pit in therecording layer whenever its intensity exceeds a predeterminedthreshold. The intensity is modulated to be alternately greater and lessthan this threshold, in accordance with the digital modulation signal tobe recorded, so that a coresponding succession of spaced pits or marksis formed in the record.

In accordance with the invention, the digitized video signal is recordedin the record 11 in a special pulse-length modulation format, in whichboth the successive marks and the spaces between successive marks havediscretely-variable lengths representative of a succession of multi-bitbinary code blocks. Digital information is thereby recorded on therecord with an improved recording efficiency.

More particularly, the recording apparatus of FIG. 1 includes ananalog-to-digital converter 21, for sampling a baseband video signalinput on line 23 and converting it to a corresponding digital signal.This digital signal is coupled over lines 25 to a formatter 27, forremoval of vertical and horizontal sync signals, compression of thedigital information, and formatting of the compressed data intosuccessive four-bit code blocks. These successive code blocks aretransfered over lines 29 to a suitable storage buffer 31, which outputsthe blocks, one by one over lines 33, to a MARK pulse-width modulator 35and a SPACE pulse-width modulator 37. The two pulse-width modulatorsoperate, in an alternating fashion, to produce output pulses havingdiscretely-variable time durations corresponding to the particular codeblocks applied to their respective input terminals. The buffer 31 musthave sufficient storage capacity to store a predetermined number of4-bit code blocks, since the blocks are input at a substantially uniformrate but are output at a variable rate determined by the particularinformation the code blocks contain.

FIG. 4 is a table showing one suitable relationship between the sixteenpossible 4-bit code blocks and the time durations for the correspondingpulses output by the two pulse-width modulators 35 and 37. It will beobserved that the possible pulse lengths vary in uniform steps between aminimum length of 1.0 L and a maximum length of 2.5 L. An alternativerelationship between the sixteen possible code blocks and thecorresponding pulse durations is provided in a copending andcommonly-assigned application for U.S. Pat. Ser. No. 974,183, filed inthe name of Jack H. Bailey and entitled "Video Player/Recorder WithNon-Linear Mark Length Modulation".

The recording apparatus of FIG. 1 further includes a combiner device 39for producing the modulation signal coupled over line 41 to theintensity modulator 17, in accordance with the successive pulse-lengthmodulated pulses received over lines 43 and 45 from the MARK and SPACEmodulators 35 and 37, respectively. The combiner also controls thetiming of the MARK and SPACE modulators by providing ENABLE signals overlines 47 and 49, respectively, initiating operation of each modulatorimmediately after the previous output pulse from the other modulator hasterminated. The modulation signal output by the combiner on line 41 isin a logical "one" state whenever the MARK modulator 35 outputs a pulse,and in the logical "zero" state whenever the SPACE modulator 37 outputsa pulse. The desired pattern of alternating marks and spaces,representative of the successive four-bit code blocks, is thereby formedin the record 11.

In the preferred embodiment, each of the successive recording tracks inthe record 11 records the digital information for a single video frame.As shown in FIG. 3, each track includes N sectors, and each sectorincludes a header portion, a data portion, and a filler portion ofvariable length. A fixed amount of data is included in each dataportion, but since each mark and space are variable in length, thelength of the entire data portion is likewise variable. The fillerportion is therefore usually necessary. In the preferred embodiment, thefiller code comprises a special sequence of marks and spaces that can beused in calibrating apparatus for playing back the recorded information.Both the header and filler portions of each sector are generated by theformatter 27 (FIG. 1), which includes registers for storing datarepresentative of the current frame and sector number and of theparticular location in the sector currently being recorded.

FIG. 3 depicts the header portion of each sector to include asynchronizing code, a sector address code, an identification codeindicating the character of the data (e.g., video, audio, etc.), anormal/complement code, and spares for permitting expansion of any ofthe previous codes. The normal/complement code is used as a specialmeans for minimizing the length of the data in each sector. Aspreviously mentioned, each sector includes a fixed amount of data, butis variable in length, in accordance with the particular code blocksbeing recorded. If it is determined by the formatter 27 (FIG. 1) thatthe required track length to record a particular sector of data exceedsa predetermined average value, then the formatter outputs the complementof each code block for recording instead, and modifies thenormal/complement code in the corresponding header, accordingly. In thismanner, the maximum track length required to store the data in anysector corresponds to the recording of marks and spaces that are all ofaverage length, i.e., that correspond to the code blocks "0111" or"1000".

FIG. 3 also depicts the format of the data portion of each sector. Itwill be observed that the data includes M successive boxels, eachpreceded by a special supermark code, for synchronization andre-initialization. In the preferred embodiment, each boxel correspondsto an 8×8 matrix derived from a segment of the video frame.

FIG. 2 depicts apparatus for playing back a video disc replica 51 of therecorded record 11 of FIG. 1. The apparatus includes a reading laser 53for producing a reading beam of light 55 having a substantially uniformintensity. This beam is focused onto the disc 51 by a radially-movablelens (not shown) as the disc is rotated at a uniform rate by a spindlemotor 57. This produces a reflected beam 59 that is modulated inintensity in accordance with the recorded pattern of marks on the disc.The apparatus then detects the modulated beam and measures the lengthsof the successive pulse-length modulated marks and spaces, to determinethe corresponding 4-bit code blocks they represent. The originalbaseband video signal is thereafter reconstructed.

More particularly, the playback apparatus of FIG. 2 includes a detector61 for detecting the modulated intensity of the reflected beam 59 andproducing a corresponding electrical signal. This signal is coupled overline 63 to a MARK pulse-width demodulator 65 and a SPACE pulse-widthdemodulator 67, which measure the lengths of the successive marks andspaces, respectively, and determine the particular code blocks theyrepresent. Each demodulator can conveniently include a linear rampgenerator that is initiated and terminated by the detected edges of eachmark (or space), along with an analog-to-digital converter forconverting the peak value of the ramp to the corresponding four-bit codeblock. The apparatus further includes a decoder 69 for interleaving thesuccessive four-bit code blocks supplied on lines 71 and 73 from theMARK and SPACE demodulators 65 and 67, respectively.

The sequence of code blocks is coupled over lines 75 from the decoder 69to a buffer and deformatter device 77, which de-compresses the datausing conventional techniques, converting it back to substantially itsoriginal digital format. Additionally, the deformatter insertsconventional digitized vertical and horizontal sync signals into thedecompressed video data. The deformatter then produces a real-timedigital video signal for coupling over lines 79 to a digital-to-analogconverter 81, which reconstructs the original analog baseband videosignal for output on line 83. The buffer and deformatter device 77 mustinclude sufficient memory capacity to store a predetermined portion ofthe successive incoming code blocks, which are received from the decoder69 at a variable rate determined by on the particular information thecodeblocks contain, while the information is being output in asubstantially real-time fashion.

It will be appreciated from the foregoing description that the presentinvention provides an improved system for recording and playing backdigital information on a disc-shaped record. The information is storedin a succession of spaced marks, with the lengths of both the marks andthe spaces between successive marks being discretely variable inaccordance with a succession of multi-bit code blocks. The digitalinformation is thereby stored with an improved recording efficiency.

Although the present invention has been described in detail withreference to its presently preferred embodiment, it will be understoodby those of ordinary skill in the art that various modifications can bemade without departing from the spirit and scope of the invention.Accordingly, it is not intended that the invention be limited, except asby the appended claims.

I claim:
 1. A method of reading digital information from an optical disccomprising the steps of:reading a succession of alternating marks andspaces recorded on the optical disc and generating a correspondingsuccession of playback signals indicative thereof; recognizing each ofthe marks and spaces when they have a length no less than a minimumlength and no greater than a maximum length, the maximum length beinggreater than double the minimum length; recognizing length differencesbetween a first length of any mark or space and a second length of anyother mark or space which are less than said minimum length; andconverting the succession of recognized marks and spaces into asuccession of binary bits indicative of the digital informationrepresented by the recognized marks and spaces in accordance with therecognized length difference, by decoding each portion of the successionof playback signals using only a first decoding operation to decode eachportion of the succession of playback signals into a portion of thesuccession of binary bits and no operation, subsequent to the firstdecoding operation, which processes the portion of the succession ofbinary bits, each decoded portion being a final code indicative of eachcorresponding portion of the succession of playback signals.
 2. A methodas in claim 1, wherein said minimum length is a value 1, said maximumlength is a value greater than 2l, and said length difference is a value0.1l.
 3. A method of reading digital information from an optical disccomprising the steps of:reading a succession of alternating marks andspaces recorded on the optical disc and generating a correspondingsuccession of playback signals indicative thereof; recognizing each ofthe marks and spaces when they have a length no less than a minimumlength and no greater than a maximum length, the maximum length beinggreater than double the minimum length, and recognizing differences inlength between a first length of any mark or space and a second lengthof any other mark or space which is less than said minimum length;dividing said succession of marks and spaces into a succession ofplayback signal portions of a predetermined length and decoding saidsuccession of playback signal portions into a succession of decodedportions using a decoding system whereby every said playback signalportion decodes into a decoded portion which unambiguously representssaid every playback signal portion, said decoding obtaining said decodedportion without additional information about portions other than theportion being decoded; and using the succession of decoded portions asthe digital information.
 4. A method as in claim 3 comprising thefurther steps of:decompressing said succession of binary bits from saidusing step to produce a succession of original binary bits; andreceiving the succession of original binary bits and generating ananalog signal representative of a succession of original binary bits. 5.A method as in claim 3 wherein each said portion of said succession ofbinary bits includes the same number of binary bits.
 6. A method ofreading digital information from an optical disc comprising the stepsof:reading a succession of alternating marks and spaces recorded on theoptical disc and generating a corresponding succession of playbacksignals indicative thereof; recognizing each of the marks and spaceswhen they have a length no less than a minimum length and no greaterthan a maximum length, the maximum length being greater than double theminimum length, and recognizing differences in length between a firstlength of any mark or space and a second length of any other mark orspace which are less than said minimum length; dividing said successionof marks and spaces into a succession of playback signal portions of apredetermined length; decoding each said portion into a decoded portionbased only on said each portion and not on a previous or subsequentportion; and using the succession of decoded portions as the digitalinformation.